<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    .box {
        width: 200px;
        height: 200px;
        border: 1px solid red;
        position: relative;
    }
    .children-box {
        position: absolute;
        width: 100px;
        height: 100px;
        background: yellow;
        left: 50%;
        top: 50%;
        margin-left: -50px;
        margin-top: -50px; 
    }
    </style>
  <title>Document</title>
</head>
<body>
  <div id="app">
    <div class="box">
        <div class="children-box"></div>
    </div>
</div>
</body>
</html>